1. Field of the Invention
The present invention relates to a hand tool comprising a body, a handle member pivotally mounted thereon and an operating member which is displaceable relative to the body between a starting position and a changeover position (where the motion of the driver changes from a lifting motion to a downward), striking motion, the displacement of the operating member from the starting position to the turning position, during use of the tool, taking place against the action of a force that increases during at least part of said displacement, and the handle member being arranged to engage the operating member so as to displace, when pivoting relative to the body, the operating member from the starting position to the changeover position.
2. Related Background Art
An example of a known hand tool of this type is disclosed in Swedish Patent Specification 8700892-6. This prior-art tool is a stapler for driving staples into an object. The operating member of the tool is a driver which engages a leaf spring and is adapted, while being displaced from a lower starting position to an upper changeover position, to tension this spring, which then produces the force counteracting the displacement. The stapler has in its lower portion a magazine which contains a horizontal row of releasably interconnected staples, which are urged forward towards a firing position in the front portion of the stapler by a spring-loaded feeding mechanism. When the driver reaches its changeover position, it is released, the spring giving the driver a striking motion starting from the changeover position in order to drive the front staple located in the firing position into an object.
The handle member of the prior-art stapler is a straight lever. The lever is pivoted to the body of the stapler by means of a horizontal pivot pin and has a short front arm and a long rear arm, which constitutes the handle of the stapler. The front arm of the lever engages by its front end in a hole in the upper portion of the driver so as to raise the driver from the starting position to the changeover position against the action of the spring when the handle, i.e. the rear arm of the lever, is urged downwards towards the body of the stapler. The lever is designed in such a manner that the front end of its front arm is moved out of the hole in the upper portion of the driver when the driver reaches the changeover position. Then the driver is released to perform its striking motion.
A recess extends transversely through the body of the stapler and is defined in its upper part by a gripping surface. The handle is to be pressed downwards by hand. The thumb of one hand is placed across the handle and the other fingers of the same hand are inserted into the recess in the body, whereupon one presses the handle downwards by making a squeezing motion with the thumb pressed against the handle and the other fingers pressed against the gripping surface. In the starting position, the handle extends obliquely upwardly and rearwardly, the opening angle being relatively great. This results in an initial state where the grasping opening of the hand (the amount that the hand must be open to grasp the tool), corresponding to the distance between the upper side of the handle and the gripping surface, is large. The grasping opening of the hand decreases during the squeezing motion and is comparatively small when the handle reaches a substantially horizontal position, in which the driver is located in its upper changeover position.
The maximum squeezing power of the hand, which henceforth is called the hand power, is relatively small in a large grasping opening as well as in a small grasping opening and is at its maximum in an intermediate grasping opening position. In the starting position of the handle, the hand power thus is fairly small and to during squeezing, i.e. with a decreasing grasping opening, will first increase to a maximum in the intermediate grasping opening position and, as the squeezing continues from this intermediate position, decrease again.
The condition described above is illustrated in FIG. 1, which is a diagram, in which the hand power as a function of the angle v for the pivoting of the handle from the starting position is shown for three persons having different strength. FIG. 1 also shows the pressing-down power which is necessary for pressing down the handle and which is proportional to the counterforce produced by the leaf spring, as a function of the angle v in the angular area between v=0 (starting position of the handle) and v=v.sub.1. The angle v.sub.1 corresponds to the horizontal position of the handle, in which the driver is located in its upper changeover position. FIG. 1 thus illustrates both the available hand power and the required pressing-down power. The spring force and, thus, the required pressing-down power increase linearly relative to the pivoting angle, whereas the hand power, as described above, first increases so as to later decrease. As is evident from the shown example, the weakest person does not have sufficient hand power to press down the handle, whereas the strongest person manages this relatively easily.
FIG. 2 shows the ratio (in percent) of the required pressing-down power to the hand power of the person of medium strength as a function of the pivoting angle v for the example shown in FIG. 1. As is evident, the person of medium strength first needs to use but a minor amount of the available hand power when pressing down the handle. Then he must gradually use a greater and greater amount of the available hand power so as to use, at the end, almost 100% thereof. Of course, this is strenuous work and also has the drawback that the precision, i.e. the ability to hold the stapler in the exactly correct position, is considerably deteriorated at the actual firing moment. Therefore a more uniform utilisation of the available hand power is desirable.